The magazine that published topless pictures of the Duchess of Cambridge is breaking strict French privacy laws but is likely to be willing to take the risk for the boost in sales.

Privacy laws in France are considered to be among the most robust in the world, with leading politicians regularly using courts to prevent the publication of embarrassing photos that would be innocuous by the standards of British tabloids.

Yet the French also have form in targeting British royals. Twenty years ago there was a war among British tabloids after the then Duchess of York, Sarah Ferguson, was pictured topless with an American businessman sucking her toes.

And in 1994 paparazzi took pictures of Prince Charles standing naked on the balcony of a French chateau in a set of photos published by the German tabloid Bild.

Thomas Roussineau, who specialises in privacy law, said that the French magazine Closer had undoubtedly broken the country's privacy laws by publishing photos of the duchess on holiday in Provence.

"It is totally forbidden," he said. "The castle is not the street, it is in a private place. And they are intimate pictures."

But he said it was likely that the magazine had weighed the potential fine against the revenue the photos would generate. "[The fine] be a very small part of the revenue they will have from these pictures," Roussineau said.

Closer's editor-in-chief defended her decision to publish. Laurence Pieau described the photos as a "beautiful series" that showed a couple in love and were in no way degrading.

French privacy laws are stricter than in the UK and have often favoured public officials trying to protect their secrets – the existence of former president François Mitterrand's second family and daughter Mazarine was only divulged to the public with his approval just before he died.

More recently the French were confronted with revelations about Dominique Strauss-Kahn's sexual proclivities and only learned of the end of the 30-year relationship between the current president, François Hollande, and socialist politician Ségolène Royal after she was defeated in the 2007 French presidential election.

"The law is more strict here," Roussineau said. "When the Dominique Strauss-Kahn story came out, I had calls from journalists in England who did not understand the reason why it was not known before.

"The reason is that the privacy law is more strict so journalists in France don't reveal this information."But Roussineau said the situation was changing. He said the fine would be around €30,000 (£24,000), compared with far larger punishments previously, which meant publications may begin to take the risk.

Politicians, the powerful and the wealthy wishing to protect their privacy benefit from one all-encompassing law in article 9 of the civil code: "everyone has the right to privacy."

There are no explicit UK privacy laws, but individuals have privacy rights deriving from the European convention on human rights, enshrined in domestic law by the Human Rights Act.

The French laws ban not only the disclosure of a person's private life but also forbid the "theft of personal image", which technically bans paparazzi taking photos even in public places.

This summer Hollande's girlfriend, Valérie Trierweiler, won a €2,000 payout after suing VSD, a celebrity magazine which published photographs of the couple walking into the sea holding hands, the state residence in the south of France.

The legal intervention astonished the French media as Trierweiler works for Paris Match, a celebrity magazine. The beach photo-shoot appeared on the front page of a number of Paris magazines, including VSD and Voici, which have pledged to fight any legal action.

The French laws are considered the strictest in Europe. Albert II, the prince of Monaco, and his family discovered that their notorious aversion to intrusions into their private lives could be big business, netting themselves and their lawyers more than €500,000 in damages in one particularly litigious year.

The French also have laws protecting people whose images are used without their permission to sell products. Nicolas Sarkozy and his wife, Carla Bruni, got a payout from Ryanair after the budget airline used photographs of them in an advertising campaign.

In 2011 Max Mosley used French privacy laws against the News of the World, after it published pictures from a video of the F1 boss at a sado-masochistic party with some prostitutes. Because the newspaper was distributed in France, Mosley won. The paper was fined €10,000 and told to pay €7,000 damages and €15,000 legal fees.

So it would appear that the French magazine, Closer, has breached the privacy of the Duke and Duchess of Cambridge and could face prosecution under criminal law for "fixing, recording or transmitting the image of a person in a private place without their consent". The maximum sanction is imprisonment and a €45,000 fine.

Gideon Benaim, a lawyer at Simkins in London, said it was time for St James's Palace to draw a line in the sand. "If I were advising the palace I would say that now is not the time for acquiescence.

"Such publishers should be pursued to the full extent permissible by the law so that they will think twice next time," he said

However Duncan Lamont, a partner in the media team at Charles Russell LLP, said the palace could face a tough legal battle to prove it was an invasion of privacy.

"It is not quite a private house. The chateau is rented out and there may be debate as to how truly private it was and whether the paparazzi were on a public thoroughfare," he said.